Referring to FIG. 1 there is shown a block diagram of a PAM modulation circuit using complex signals. The diagram mathematically highlights the steps taken in modulating the input bits (bK). The incoming bit stream bK is provided to a coder, that converts the “0” and “1” digital bits into a stream of complex symbols (am). Since the coder may map multiple bits into a single data symbol, a distinction has to be made between the symbol rate and the bit rate. In communication systems such as the BLUETOOTH™ or the Global System of Mobile communications (GSM) compliant systems, there is a one-to-one correspondence between the bits and symbols: {0,1}→{−1,1}. More advanced encoding schemes, such as QPSK, for example, pack two bits into a symbol.
Symbols are applied to a transmit filter, which normally produces a continuous-time signal for transmission over the continuous-time channel. The impulse response g(t) of the transmit filter is called the pulse shape and can be gaussian or raised-cosine. In modern implementations, the pulse shape is oversampled by a chip clock and represented digitally throughout the pulse filtering process, even though the filter output s(t) is usually brought back to the continuous-time domain by performing a digital-to-analog conversion (DAC) and subsequent low-pass filtering.
The digital baseband data bits bK are synchronous to the baseband clock whereas the digital filter output samples are synchronous to the chip clock. Complex signal representation requires two physical wires that carry both real-valued parts of a complex number. FIG. 2 shows another prior art block diagram that highlights another PAM transmit modulation technique using in-phase (I) and quadrature (Q) signals, that represents a natural progression towards a more physically-realizable representation as compared to FIG. 1. Finally, in FIG. 3, there is shown still another block diagram of a PAM transmit modulation circuit that uses direct digital amplitude and phase modulation. The direct phase modulation is performed by modulating the oscillator tuning input in a feed-forward manner. The direct amplitude modulation may be performed by regulating the supply voltage to a constant-envelope power amplifier.
Prior art envelope elimination and restoration PAM transmit modulation methods that adjust the supply voltage of a non-linear amplifier according to the desired amplitude, although useful, require a lot of semiconductor area to implement. This is because these circuits typically require an envelope detector, a DC-DC converter, as well as an amplitude limiter. These PAM circuits tend to suffer from impedance mismatching problems for lower amplitude output signals, and are sometimes power-inefficient from a power-added efficiency standpoint.
Still another PAM transmit modulation method used in the prior art is called Linear Amplification with Nonlinear Components (LINC). LINC techniques add two constant-envelope power amplifier (PA) outputs of properly phase-shifted signals together. LINC techniques tend to be semiconductor area intensive and power inefficient, and are thus mainly used in fixed communication equipment such as base stations, etc. A need exists in the art for a PAM transmit modulation technique that minimizes some of the problems mentioned above, so as to be useful not only in fixed communication equipment but also portable communication equipment.